In corrosive and high temperature environments, heat exchangers made of monolithic materials such as graphite are routinely used in the chemical industry. Similar designs using other ceramic materials are also used, though less frequently due to cost and size limitations. In small heat exchangers, the entire unit may be one block of solid material with holes bored through in cross flow patterns known to those in the industry. Larger heat exchangers are usually assembled from multiple blocks where at least one of the fluids makes multiple passes in a zig-zag pattern flowing at substantially right angles to the other flow allocation. The second flow allocation can be a series of passes, or straight through the long axis of a heat exchanger. In most cases, the multiple block designs are due to size limitations of the starting monolithic materials.
Heat transfer is most efficient and cost effective when countercurrent flow can be employed in compact designs keeping opposing flow channels as close to each other as reasonably possible. Cross flow rather than counter current flow represents the current state-of-the-art in virtually all commercial scale heat exchangers made of monolithic materials. In the majority of existing cross flow designs, external containment made of multiple metallic components is used to route the fluid allocated to at least one side of the heat exchanger through its multiple passes. These designs are mature and work well when the cross flow fluid is not particularly corrosive (i.e. such as cooling water, refrigerant, or steam) or at temperature extremes. In heat exchangers with many cross flow channels and large temperature differences between the fluid allocations, the cross flow design can approach the thermal performance of a counter current design when substantially cubic blocks are used.
In processes where both fluids are corrosive, or at high temperature such that metallic components cannot be exposed to either fluid, and when for fabrication or process reasons such as high pressure it is desirable to use assemblies of substantially cylindrical blocks rather than substantially cubic or rectangular blocks, there is a long sought and unresolved need for a better design.
Chemical vapor deposition (CVD) reactors are used to produce polycrystalline silicon (polysilicon), the key raw material used in the manufacture of most semiconductor devices and silicon-based solar wafers and cells. The most widely used method for producing polysilicon is the Siemens reactor process, which has been in existence for about fifty years. In this process, high temperature polysilicon rods are placed in a reactor, and trichlorosilane (TCS) gas is passed over these rods. The silicon in the gas is deposited on the rods, and when the rods have grown large enough, they are removed from the reactor. The end product is in the form of polysilicon rods or chunks, which can be further processed into ingots, then sliced into wafers that are made into solar cells, for example. In a related process, TCS is disproportionated to form silane (SiH4) and STC. The silane produced is used in many processes associated with semiconductors and other products, including making polysilicon in either a Siemens reactor or fluidized bed CVD process. The fluidized bed process makes silicon in irregular, but nominally spherical beads in diameters typically ranging up to about 2 mm diameter.
The process converting TCS into silane and the CVD-based Siemens process for manufacturing polysilicon both produce a large amount of the byproduct silicon tetrachloride (STC). For example, a maximum of about 20 kg of STC is made as a byproduct for every kg of polysilicon or silane produced. It is possible, however, to hydrogenate STC forming TCS by reacting STC with hydrogen in the gas phase at high temperature. The product TCS can then be recycled to a series of silane disproportionation reactors and separation steps to make silane, or to a CVD reactor for direct production of more polysilicon. If STC could not be recycled, there would be a huge loss of the primary raw materials silicon and chlorine and a cost for disposal of the byproduct STC.
To efficiently react STC with hydrogen to form TCS, high reactant gas temperatures (e.g., greater than 850° C.) are required. Current commercially available systems sold for conversion of STC to TCS use retrofitted Siemens style CVD reactors with electrically heated graphite rods to heat the reactant gases. This equipment has a number of problems. For example, because CVD reactors have a high volume to heated rod surface area ratio, the local velocities and the heat transfer coefficients in the reactor are low. Thus, very high rod surface temperatures are required (e.g., temperatures greater than 1400° C.) to heat the reactant gas to sufficient temperature. Furthermore, the retrofitted CVD reactors have a large, heavy baseplate, that is expensive and makes it inconvenient to add heat exchanger equipment for recovery of heat.
Moreover, the heated graphite rods in a retrofitted CVD reactor require a large number of electrical connections. For example, the reactor may require up to 24 U shaped heater rods with up to eight electrical connections per hairpin. Every connection is a potential source of rod failure. Every baseplate penetration represents risks for electrical ground faults.
Furthermore, CVD reactors have a high radiation heat loss to the shell, wasting large amounts of energy. Insulating materials can be installed inside a Siemens style hydrogenation reactor. Such insulation must be installed around the outer perimeter of the heating rods requiring a substantial quantity of material. The insulation itself will heat nearly to the temperature of the heating rods requiring use of expensive materials in order to achieve long life. Cheaper insulating materials do not exhibit an adequate lifetime due to reaction with reactant gases at the high temperatures involved. As a result of the size and expensive nature of the materials needed, insulation on a Siemens style hydrogenation reactor is very expensive. Some Siemens style hydrogenation reactors feature a primitive heat exchanger for heat recovery. With the use of insulation and a primitive heat exchanger, a retrofitted CVD reactor for conversion of STC to TCS requires energy of at least 1.5 Kwhr per kilogram of TCS manufactured, which is quite high. With no heat exchanger at all, a retrofitted CVD reactor requires up to 3.5 Kwhr/per kilogram TCS produced or even slightly more if the thermal losses are great. Key components of the hydrogenation reactor have limited lifetimes and must be replaced at regular intervals—including the heating elements, the electrical connections, the insulation, and components of the heat exchanger.
Purpose-built (non-retrofitted) systems for conversion of STC to TCS have been proposed and are the subject of patents by Mitsubishi, Wacker, and Hemlock Semiconductor, which promise to be more energy efficient and cheaper to build than retrofitted CVD reactors (see, for example, U.S. Pat. Nos. 5,906,799 and 7,998,428; U.S. Patent Application Publication 2011/0215084; International Publication Nos. WO/2006/081965 and WO/2010/116440; and European Patent Application Publication Nos. 2 000 434 A1 and 2 088 124 A1). However, such purpose-built systems are not widely used, and are not yet commercially available. U.S. Pat. No. 7,442,824 describes a purpose-built STC to TCS hydrogenation reactor with heating elements and a reactor wall that are coated with silicon carbide (SiC) to prevent contamination and degradation of these components in high temperature reaction environments. The hydrogenation reactor employs graphite heating rods, as used in retrofitted CVD reactors creating numerous connection points for potential electrical and mechanical failure. U.S. Pat. No. 7,964,155 shows a substantially different approach to an STC hydrogenation reactor where gas flows from outside diameter to inside diameter through a series of concentric cylindrical baffles forming a heat exchanger with a heating element surrounded by the heat exchanger. This design appears challenging to fabricate and limited in scalability and heat transfer efficiency as the gas must flow from a large to a small diameter.
Thus, there is a need for a more efficient STC to TCS hydrogenation reactor suitable and available for commercial use.
All patents, patent applications, provisional patent applications and publications referred to or cited herein, are incorporated by reference in their entirety to the extent they are not inconsistent with the teachings of the specification.